Here's the deal: You don't need to withdraw and return the current security deposit just to collect the additional $100.

Yes, you can deduct the 8 days rent from the security deposit.

The pet fee or deposit for the move in of 2 cats (you did charge one, I hope) should have just about covered the extra anyway.

In this way, I never need to be concerned about justifying my charge to their deposit because the damage is clear and obvious. 

Now, over a year and a half later, he sent a certified letter last week demanding the security deposit or legal action within 30 days.Please advise. 

Paint rags and paint plastic $386.66 Initial 70% deposit Cabinets $13,029.02 Backer board, Fiber rock, Shower drains, Zinc backer board screws $385.71 Junk Removal $600.00 red gard ($123.10) Initial 50% deposit Windows $10,493.20 2x4 studs $52.33 Junk Removal $400.00 Flooring sample $6.01 Flooring sample $68.48 Flooring sample $62.62 Flooring sample $132.09 Paint samples for accent wall $38.34 10% Draw for Master bathroom demo $580.00 Wall and shower Tile+Grout-Guest bath $528.62 LVP Flooring $7,165.27 Demo Work $1,275.00 LVP Flooring $7,198.07 Floor tile-Guest bath $889.05 50% down for moisture barrier and animal extraction $2,541.30 Returning flooring samples ($263.19) Paint-2 5gallon buckets SW $382.38 Unsure on what these are $19.54 Unsure on what these are $48.00 Master shower floor tile $331.42 Paint $3,100.00 Flooring underlayment $888.42 Primer $178.71 Paint-5 1gallon buckets SW $180.26 Paint $3,100.00 Appliances $6,820.59 Master bath tub $986.76 Paint-1 5gallon buckets SW $194.81 Primer $268.54 Framing repair for rotted wood $1,000.00 Junk Removal $500.00 Floor tile-Guest bath $771.07 Floor and Shower wall tile-Master $4,559.43 Stair noses $334.20 Master shower floor tile $612.76 Farmhouse sink $385.91 Master ceiling paint $254.29 Drywall repair in kitchen ceiling $500.00 Childrens wall tile and floor tile $3,477.40 Master shower floor tile-Return ($331.43) Drop in sinks for children's bathroom $103.75 Drop in sinks for upstairs bathroom $51.87 Master sinks $147.64 Master floor tile $270.05 Electricity $169.70 16x cement backer board return ($139.60) Tub and shower faucets $414.96 4 x Toilet seats $788.22 Tile install materials for all bathrooms $863.00 Air filters $10.90 Gas pipe and AC tune up+Duct repair $1,216.00 Tile install materials for all bathrooms $984.78 Roll of insulation X2 $38.48 Primer refund $125.33 Shower faucets X2 $480.71 Water $11.41 Master shower floor tile-Return the original white one ($261.55) Demo and materials he bought $1,005.14 Pickup charge for picking up flooring $223.96 Flooring 1st payment $4,000.00 Flooring return-guest bathroom tile ($889.05) Red guard refund $71.34 Backer board, wall tile and floor tile upstairs master return ($134.34) Keys X3 $10.93 Remaining balance for moisture barrier $2,046.30 Subfloor repair $100.00 Flooring $9,900.64 Master tub filler faucet $311.35 Tile labor in all bathrooms $9,202.22 Stair treads and risers $1,160.76 Bathroom painting $2,000.00 Junk Removal $150.00 Return for stair noses ($334.20) Return for flooring ($13,488.73) Flooring $1,900.00 Flooring underlayment $899.35 Trim $1,053.17 Toilet supply lines $13.46 Tile sealer, garage epoxy, garbage disposal $313.00 Return for remaining balance ($1,098.57) Floor reducer $49.99 Garage door opener $267.66 Door hardware (Hinges and door handles) $451.14 Counter tops $5,287.01 Tile return ($981.43) Tile return ($685.14) Tile return ($385.53) Tile return ($153.19) Tile return ($613.13) Flooring $1,900.00 Plumbing and electrical work (Light fix, toilets, drains) $975.00 Light fixtures and faucets $975.49 Junk Removal $125.00 Baseboards $241.45 Switches, outlets, faceplates $234.53 Cabinet balance $5,583.86 Sink return ($155.62) Dining room light and 4 ceiling fans $923.70 Anti skid additive for garage floor $11.38 Flooring $1,900.00 Glidden gel paint $24.01 Paint refund ($1,046.40) Counter tops $2,304.23 Extension rods for ceiling fans $40.64 3 Ceiling fans (one was returned) $573.93 Return grills $100.11 Kitchen and nook light fixtures $283.80 Rod for curtains and toilet paper holder $84.10 Curtain rods $54.60 Garage door deposit $1,185.00 Cleaning $400.00 Water heater $756.18 Thermostat $181.41 Stump Removal $300.00 Dryer cord $28.53 Plumbing and electrical work (Light fix, toilets, drains) $3,525.00 Junk Removal $125.00 Grass seed $71.40 Return of shipping cost ($297.45) Duct cleaning $970.00 Drains for master $46.54 Sold Fridge ($200.00) Bathroom fans $91.92 Samples for brick $29.98 Peg board hooks $20.83 Wall plate covers $21.28 Steel pegboards $140.72 Return stair tread and Riser ($198.14) Pressure washing $1,216.00 Wall plate covers $8.11 Shower door for upstairs $729.83 Powder room counter top $702.99 Light bulbs $59.24 Shower door deposit $1,000.00 Staple gun, staples, supply lines $157.04 Bathtowel hanging hardware $130.89 Light bulbs $29.62 GFCIs, underlayment, downrods ($233.54) Sanding pads $59.10 Yard hanging tools $21.94 Supply line return ($13.12) Stair clips and balusters $779.58 Returned some bathroom tile ($109.05) Peg board hooks $120.04 Front door $3,235.01 Brick sealer $50.81 Bathroom fan light and peg board hooks $71.27 Sanding pads, sanding sheets, etc $38.48 Mirrors $65.84 Mirrors $291.88 Spray bottle for marble sealer $4.34 Brick backsplash $695.54 Sanding pads $2.20 Shelves and shelf brackets for storage room $73.14 Shower door balance $1,000.00 Mirrors $104.26 Returned mirrors ($89.50) Gutters $2,043.15 Floors for finishing room $373.78 Wall plate covers $8.07 Floetrel and hood vent $31.49 Plumbing and electrical work $175.00 Backsplash and fireplace $2,450.00 Trim paint $50.21 Tile for master finish $15.17 Returned something.

If it's not a holiday, I would say, "No, the deposit remains in place until your lease ends."

No free weeks, no reduced deposit, etc.

Quote from @Phillip Austin: There are two alternatives to a security deposit, which I speak about in my book.

You can create an account to advertise yourself as a host or a guest (or both) and get matched with others.